This invention relates to an antenna unit and, in particular, to structure of a shield cover of the antenna unit used as a Global Positioning System (GPS) antenna.
In the manner which is known in the art, in recent years, various antennas are mounted on a vehicle such as an automobile. For instance, there is a GPS antenna as one of the antennas.
The GPS (Global Positioning System) is a satellite positioning system using artificial satellites which are called GPS satellites. The GPS is a system which receives radio waves (GPS signals) transmitted from four GPS satellites among twenty-four GPS satellites orbiting the Earth, measures, on the basis of the received radio waves, position relationships and time errors between a mobile object and the four GPS satellites, and can calculate, on the basis of the principle of triangulation techniques, a position and/or an elevation of the mobile object on a map with high precision
In recent years, the GPS have been used to a car navigation system for detecting a position of a running automobile or the like and becomes widespread. In the car navigation system, a car navigation apparatus comprises a GPS antenna for receiving the GPS signals, a processing unit for processing the GPS signals received by the GPS antenna to detect a current position of the vehicle, a display unit for displaying, on a map, the position detected by the processing unit, and so on.
On mounting the car navigation apparatus in the automobile, the GPS antenna for receiving the GPS signals is mounted outside of the automobile, for example, on a roof of the automobile or the like (see, for example, United States Patent Application Publication No. 2005/0068247 A1 which will later be called a patent document 1 hereinafter).
An antenna unit such as the GPS antenna comprises an antenna case and an antenna module accommodated in the antenna case. The antenna module comprises a circuit board having a main surface and a back surface, a planer antenna element such as a patch antenna mounted on the main surface of the circuit board, a low noise amplifier (LNA) circuit mounted on the back surface of the circuit board, a shield cover for shielding the LNA circuit.
The shield cover has, as a shape of a plan view, a substantially rectangular shape with four corners each of which is chamfered. That is, the shield cover has four corner portions at the four corners. In addition, the shield cover is mounted on the back surface of the circuit board by soldering the four corners (the four corner portions) by means of solder.
In the manner which will later be described in conjunction with FIGS. 1A, 1B, and 2, a conventional shield case comprises a ceiling portion, four side wall portions, and four corner portions. However, in the conventional shield cover, there are gaps at folded portions of the shield cover (i.e. between the four side wall portions and the four corner portions). As a result, on mounting the shield cover on the back surface of the circuit board by soldering, solder is flowed in the inside of the shield cover through the gaps and it results in danger of causing a short in the LNA circuit mounted in the inside of the shield cover.
In addition, a shield case for shielding a circuit board housed therein is known (see, for example, Japanese Unexamined Patent Publication No. Hei 8-293688 or JP-H 8-293688 A which will later be called a patent document 2 hereinafter). The shield case disclosed in the patent document 2 is for shielding the circuit board housed therein and is different in structure from the shield cover mounted on a back surface (one surface) of a circuit board in order to shield a circuit such as an LNA circuit mounted on the back surface (the one surface) of the circuit board.